The invention relates to a method of manufacturing a rotationally symmetrical flared annular core of oxide ceramic ferromagnetic material suitable for a television deflection unit in which a moulding consisting of a compressed starting material is sintered to form a ferromagnetic core. The invention also relates to a ceramic holder for use in a method of manufacturing annular cores.
Annular cores of ferromagnetic material are used in T.V. deflection units. Such annular cores may be conical or frusto-conical. The inside diameter of the rotationally symmetrical annular core, viewed in the axial direction, increases from the rear side of the core to its front which is to face the display screen. It is usual to manufacture such cores from an oxide ceramic material, for example ferrite. Ferrite cores are manufactured by mixing ferrite-forming starting materials, for example oxides of iron (Fe.sub.2 O.sub.3), of nickel (NiO) and of zinc (ZnO), by compressing the oxide mixtures, with or without the use of a binder, into moulds and sintering these mouldings. When the annular core consist, for example, of nickel-zinc-ferrite, sintering is carried out at a temperature of approximately 1,000.degree. to 1,400.degree. C. During sintering a so-called solid state reaction occurs so that the metal oxides are combined to form a spinel lattice which causes the ferromagnetic properties. At the same time the moulding is sintered to form a mechanically rigid ceramic body.
It has been found that the annular cores manufactured in this manner often show large or small cracks after sintering and subsequent cooling, in particular in curved places where the inside diameter or outside diameter increases considerably and which may even cause fracturing of the annular core in the case the cracks are circumferential.
In German Patent Specification No. 1,196,109 the occurrence of these cracks is ascribed to an inhomogeneous concentration of the moulding during the compression process and it has been suggested to replace the compression by a centrifuging process in a magnetic field. Although this measure in itself seems to make sense, it has been found that an annular cores which have been incorporated in ceramic holders during the sintering process cracks may also occur when the moulding has been very homogeneously concentrated. These holders (so-called "saggars") are necessary to be able to stack the annular cores when the furnace in which the sintering process is carried out is to be filled economically. Such holders have the form of a cup having a bottom in which a central aperture is provided. The annular cores are "suspended" in these apertures during the sintering process. Another disadvantage is that the annular cores fired in this manner sometimes show a certain unconcentricity.